The present invention relates to a tennis racket of a type in which frame and handle shaft are integrally made of fiber reinforced plastics (hereinafter referred to as "FRP") and the frame is strung with some strings such as a gut. 2. Description of the Prior Art
In recent years, tennis rackets having enlarged frames such as so-called large-size racket and mid-size racket have been broadly used. These rackets can provide some advantages in comparison with tennis rackets having conventionally sized frames. In detail these new type rackets provide a high rebounding coefficiency and enlarge their sweet spot. On the contrary, such larger rackets tend to be twisted considerably when an user hits a ball at a point outside of the sweet spot. This tendency is particularly remarkable in the case of FRP-made rackets rather than wood or aluminum-alloy made rackets, because of the small specific gravity of FRP material.
In order to reduce the twisting of rackets, it has been known effective to increase moment of inertia in their frames, and some proposals have been made to attain a relatively large moment of inertia. For example, Japanese Utility Model Applications as laid-open under No. 54-41364 and No. 61-127766 show the tennis rackets in which a metal weight is fixed to a portion of the frame. However, such racket involves new problem. That is, additional component such as the metal weight is not integrally formed with the main component which is used for the frame per se, and thus such different components concentrates mechanical stress on a particular portion so that the frame may be broken in the vicinity of the weight.
Also, Japanese Utility Model Application No. 58-188069 shows another example of improved racket whose frame is added with weight by protruding a part of the internal circumferential surface of the frame radially inwards toward the center of strung surface. However, this racket also causes several problems owing to its frame shape. Such protruded section increases an air resistance and generates turbulent flow which is not ignored, so that energy loss is increased during swing and user may feel unpleasant.
It is therefore an object of the present invention to provide a tennis racket of which frame can have an increased moment of inertia for reducing twisting of the racket, without increasing a surface area of the frame in a plane of a strung surface.
Another object of the present invention is to provide a tennis racket which can effectively transmit an energy to a ball and can provide a good swing feeling.
Still another object of the present invention is to provide a tennis racket improved in its mechanical strength and durability.
A further object of the present invention is to provide a tennis racket which can be manufactured in a simple work.